I did some calculations that a solar panel on the roof, tied into the traction battery in some fashion (whether it's directly being used for miles or offloading climate systems) would add maybe 3 miles range to the car. That's GAS saved, not electricity, so about 10¢/mi , or 30¢ per day. It would take 5 years saving 30¢ per day to pay back an investment of $500, so that was my vote.
But realistically, I park indoors at home and work, so I'd except only about 30 minutes to an hour on the road each day, It wouldn't generate much electricity at all for me... so even the estimate above is way over the top.
3 miles per day was based on my Whr/mi rating and some stats I got from an Energy Del Sol solar mat for RVs. It produces 68W from a 13.5 sq ft pad. There's probably better grades of PV cells out there, but it was a start.